1. Field of the Invention
This invention relates to a semiconductor integrated circuit device, specifically relates to a device with a boost circuit for generating an internally boosted voltage in the chip.
2. Description of the Related Art
There have been provided various types of electrically rewritable and non-volatile semiconductor memory devices (EEPROMs), typical one of which is a NAND-type flash memory. In these semiconductor memories, various kinds of internally boosted voltages are generated in the chip, which are necessary for write, erase and read operations.
An internal voltage generating circuit is basically formed of a boost circuit for boosting power supply voltage and a voltage detecting circuit (i.e., voltage limiter) for detecting the output voltage of the boost circuit to control ON/OFF thereof so as to keep the output voltage at a certain level (refer to, for example, Unexamined Japanese Patent Application Publication No. 11-353889). The boost circuit is usually formed of a charge pumping circuit and a clock generator for driving it.
For example, in the NAND type flash memory, one page data write is performed at a time. In this case, to precisely control the threshold distributions of write data, it is used such a write sequence that write voltage applications and write-verify operations are repeatedly performed.
In the above-described write sequence, for example, paying attention to a Vpgm generator in the internal voltage generating circuits, which applies a write voltage Vpgm to a selected word line, a voltage detecting circuit monitors the output voltage of a boost circuit to control ON/OFF thereof, so that the write voltage Vpgm is generated with a necessary level through the write sequence.
However, the write voltage Vpgm is used in practice only during the write voltage applying period, i.e., only when it is supplied to a selected word line as a load. During the verify operation, the Vpgm generator operates intermittently for compensating the through current of the voltage detecting circuit. The voltage detecting circuit is formed to have a resistive voltage divider circuit, in which a consumptive current flows.
The write voltage Vpgm is set at about 20V. To boost an external power supply voltage of 3V or lower to the above-described level, it is required of the charge pumping circuit to contain many boosting stages (i.e., transferring stages), so that power consumption thereof becomes large.
Therefore, it becomes a cause of increasing the power consumption of the memory chip that the voltage detecting circuit continues to carry a current, and the boost circuit continues the intermittent operations. Specifically, in case a large capacitive flash memory is adapted to mobile devices, it becomes an important subject to reduce the power consumption.